International Journal of Engineering and Technologies
ISSN: 2297-623X, Vol. 8, pp 32-43
doi:10.18052/www.scipress.com/IJET.8.32
CC BY 4.0. Published by SciPress Ltd, Switzerland, 2016
Submitted: 2016-03-27
Revised: 2016-07-14
Accepted: 2016-07-21
Online: 2016-08-09
The Effect of Heat Treatment on the Mechanical Properties
of SAE 1035 Steel
Obiukwu Osita1, a *, Udeani Henry 2, b, Ubani Progress3, c
1,2,3
Department of Mechanical Engineering,
School of Engineering and Engineering Technology,
Federal University of Technology, Owerri, Imo State, Nigeria.
a
obiukwu@futo.edu.ng, bobinech@yahoo.com, cobinech@gmail.com
Keywords: Heat treatment, fatigue, Impact, Hardness, Tensile test, SAE 1035 carbon steel.
Abstract. The effect of various heat treatment operations (annealing, normalizing, tempering) on
mechanical properties of 0.35% carbon steel was investigated. The change in the value of endurance
limit of the material as a result of the various heat-treatment operations were studied thoroughly. It
was found that the specimens tempered at low temperature (2000C) exhibited the best fatigue
strength. Microscope was used to characterize the structural properties resulting from different heat
treatment processes. The results from the tensile tests impact tests and hardness tests showed that
the mechanical properties variate at every heat-treatment conditions. The microstructure of
differently heat-treated steels was also studied.
Introduction
Engineering components and structures are regularly subjected to cyclic loading and consequently
are prone to fatigue damage, which in most cases start at the surface due to localized stress
concentrations caused by machining marks, exposed inclusions or even the contrasting movement
of dislocations [1]. As technology has been developed, fatigue has become more prevalent in
automobiles, aircraft, turbines, etc. subject to repeated loading and vibration. It is estimated that
fatigue is responsible for 80% to 90% of all engineering failures [2]. According to [3] fatigue failure
involves a multi-stage processes that begins with crack initiation, followed by a progressive crack
growth across the part with continued cyclic loading, and finally the sudden fracture of the
component or specimen. [4]. Fatemi and Yang [2] reported that there are three commonly
recognized forms of fatigue namely; high cycle fatigue (HCF), low cycle fatigue (LCF) and thermal
mechanical fatigue (TMF). Improvements in fatigue performance in components are derived
primarily by decreasing the surface cyclic tensile stress or by increasing the surface yield stress,
thereby increasing the resistance to fatigue crack nucleation. To achieve these goals, common
surface modification processes, which often simultaneously increase the surface yield stress and
introduce a residual compressive stress to decrease the surface cyclic tensile stress, are based on
heat treating. Wei et al. [5] studied the fatigue behavior of 1500 MPa bainite/martensite duplex
phase high strength steel. It was observed that fatigue strength increases and fatigue crack threshold
gives lower crack propagation. The steel produces a better combination of strength, toughness and
fatigue properties when tempered at 370C for 2 hours. Mechanical properties are enhanced as the
materials gone through the heat treatment processes [6]. In Zahid et al. [6], specimens
corresponding to all heat treatment temperatures showed higher hardness as compared to the
annealed specimens of the same steel. In general, quenching and tempering results the optimum
fatigue properties in heat treated steels although at a hardness level above about Rc 40 bainitic
structure produced by austempering results in better fatigue properties than quenched and tempered
structure with the same hardness [7]. The poor performance of the quenched and tempered structure
indicated by electron micrographs is the result of stress concentration effects of the thin carbide
films which are formed during the formation of martensite in tempering and also the fatigue limits
increases with decreasing tempering temperature up to a hardness Rc 45 to Rc 55 which is well
explained [8]. Fatigue properties at high hardness level are extremely sensitive to the surface
This paper is an open access paper published under the terms and conditions of the Creative Commons Attribution license (CC BY)
(https://creativecommons.org/licenses/by/4.0)
International Journal of Engineering and Technologies Vol. 8
33
preparation, residual stresses, and inclusions. Only a small amount of non-martensitic
transformation products can cause an appreciable reduction in fatigue limit [9]. The effect of
intercritical heat treatment on the mechanical properties of AISI 3115 steel was investigated by
Maleque [10]. The experimental results showed that tensile strength increases but impact strength
decreases with increasing intercritical temperature, correspondingly with the increase in amount of
martensite in the steel. Influence of cold rolling threads before or after heat treatment on high
strength SI grade 12.9 bolts for different fatigue preload conditions was carried out by Horn [11].
Hence, steels are normally hardened and tempered to improve their mechanical properties,
particularly their strength and wear resistance. In hardening, the steel or its alloy is heated to a
temperature high enough to promote the information of austenite, held at that temperature until the
desire amount of carbon has been dissolved and then quenched in a particular medium at a suitable
rate [12]. Also, in the hardened condition, the steel should have 100% martensite to attain maximum
yield strength, but it is very brittle too and thus quenched steel is used for very few engineering
applications. By tempering, the properties of quenched steel could be modified to decrease hardness
and increase ductility and impact strength gradually [13]. The resulting microstructures are bainite
or carbide precipitate in a matrix of ferrite depending on the tempering temperature [14]. In this
study the effects of selected heat treatment techniques on steel mechanical properties, as controlled
by processing, is presented.
Experimental Procedures
Sample Preparation. 0.354 % carbon steel was selected as a material to study the effect of heat
treatment on its fatigue strength. Composition of the steel is summarized in Table 1.
Table 1. Chemical composition of as received Medium Carbon Steel
C
Si
S
P
Mn
Ni
Cr
Mo
V
W
Cu
Al
Ti
Fe%
0.354 0.236 0.061 0.052 0.730 0.098 0.158 <0.0001 <0.0001 <0.0001 0.292 0.021 0.008 97.99
Source: Spectroanalysis done at Universal Steels Limited, Lagos
The specimen, which was received in ribbed form, was firstly machined to a standard configuration
of 10mm and 50mm gauge diameter and length respectively as shown in Fig. 1 and Fig.2. A set of
specimens was prepared for mechanical tests and microstructural analyses. Tensile test and other
mechanical specimens were also produced from the as-received medium carbon steel samples of the
same composition.
Fig.1. A machined specimen of SAE 1040
Fig. 2. A Sample of the specimen
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Heat Treatment. Standard heat treatment procedures were adapted to heat treat the carbon steel.
Five different samples were prepared for each of the operation. Some of the conventional heat
treatments procedure chosen include: annealing, normalizing, tempering and hardening test.
Annealing. The specimen was heated to an annealed temperature of 8500C. At 8500C the samples
was held for 1 hour to ensure thorough homogeneity, then the furnace was switched-off so that the
furnace and samples temperature gradually decrease to room temperature. The specimen was taken
out of the furnace after 48 hours of gradual loss of heat when the furnace temperature would have
attain the nominal room temperature.
Normalizing. Each samples of the medium carbon steel to be normalized were placed in the
furnace and heated to temperature of 850°C, was also soaked at that temperature for 1 hour, then
allowed to cool in air at a controlled rate- Meanwhile another set of the sample specimens which
were not heat treated were taken directly for the tensile test to serve as control samples.
Hardening. The selected samples were heated to 9500C. At 9500C the samples were held for 1 hour
to ensure uniform homogeneity. In order to enhance the hardness, the red hot steel is directly and
rapidly cooled. Pair of two samples was cooled in automobile engine oil while another pair of two
was cooled in water that is at an elevated temperature 40 0C. This is to avoid quench crack that
could ensue if it were cooled at room temperature or below.
Tempering. After quenching the samples from red hot condition to a temperature above room
temperature, the samples were subsequently re-heated in the muffle furnace to 200 0C, held for 30
minutes and then air-cooled in order to toughened it and improve on the ductility of the specimens.
Hardness Test. The heat treated specimens were now subjected to hardness test. The hardness of
the as received and heat-treated specimens was evaluated using a Brinell hardness (LECO AT700
Micro hardness Tester). Multiple hardness tests were performed on each sample and the average of
the best values taken as a measure of the hardness of the specimen.
Tensile Testing. After the successful heat treatment operation, the various heat treated samples
were taken for the tensile test. The test was performed on Standard Universal Testing Machine.
Tensile tests were conducted at various strain rates of 200, 500, 1000, 1500 and 1650 mm/min for
all the specimens.
Fatigue Test. The fatigue test was done using Avery Denison machine (Avery model 7305). The
bending moment was measured. The revolution counter fitted to the motor records the number of
stress cycles to failure (N). The calibration curve supplied with the machine was used to show the
relationship between dial gauge reading and the imposed torque. The essence of this is to determine
the actual load the material in question can withstand before failure in service. The stair case
method was used in applying the moment. The applied bending moment was increased by a fixed
increment and the next specimen was tested with the new bending moment. The fatigue test of the
as received and heat-treated specimens were subjected in to a fatigue testing machine and the
specimen were fitted on the two grips tight before the machine start. When the specimen breaks the
readings were collected. Fig. 3 shows the Avery Dennison fatigue testing machine. The bending
moments imposed were 604, 1208 and 1812.3 MPa for various heat treatments. The bending test
was performed at a frequency of 50Hz (1400rpm) for each specimen. It was a complete reversed
cycle of stress range (R) and is equal to minimum stress divided by maximum stress which is equal
to a negative value (-1) in fatigue tests.
International Journal of Engineering and Technologies Vol. 8
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Fig. 3. Avery Denison Fatigue testing machine
Microstructure Examination. Microstructure examination of the as-received and heat-treated
specimens was carried out using the Accuscope microscope. Each sample was carefully grounded
progressively on emery paper in decreasing coarseness. The grinding surface of the samples were
polished using Al2 O3 carried on a micro clothe. The crystalline structure of the specimens were
made visible by etching using solution containing 2% Nitric acids and 98% methylated spirit on
the polished surfaces. Microscopic examination of the etched surface of various specimens was
undertaken using a metallurgical microscope with an inbuilt camera through which the resulting
microstructure of the samples were all photographically recorded with magnification of 400.
Results and Discussion
Microstructural Analysis of the Specimen. Fig. 4 to Fig. 8 show the microstructural view of the
specimens. The effects of heat treatment on the microstructures of the samples were studied using
the Accuscope microscope. It was observed that the morphologies of the as received samples
changed with the heat treatment processes. Some grains are noticed within structures of the samples
as the austenitic temperature increased. This significantly alters the orientation of the grains in these
samples and it was expected that this change will affect the behavior of these samples when heat
treated. From fig. 4, it can be seen that the microstructure comprises of the ferrite (light areas) and
pearlite (dark areas) phases of the steel sample. The grain boundaries are even hardly visible due to
homogeneity of the constituents in the material.
Fig. 4. Microstructural view of the as-received specimen at 360X
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Fig. 5. Microstructural view of the Hardened specimen at 360X
`
Fig. 6. Microstructural view of the Tempered specimen at 360X
Fig. 7. Microstructural view of the Normalized specimen at 360X
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Fig. 8. Microstructural view of the Annealed specimen at 360X
As shown in figures 7 and 8 at the 360X magnification, the microstructure of normalized and
annealed samples comprises of both ferrite and pearlite in equal proportions. Their grain boundaries
are not well defined but the grain boundaries of the annealed samples are coarser than that of the
normalized one. The Figures reveals visible phases present in the steel and cracks are clearly visible
along the grain boundaries of the sample. This is an indication that the heat treatment of the sample
subjected it to some internal stresses which have caused cracks within the phases of the material.
The quenching and tempering heat treatment process caused the formation of scattered grain
particles which spread through in transgranular and intergranular spaces of the materials.
Microstructural inspections indicated that the surfaces of tempered samples are martensitic.
Hardness Result. The results obtained from the hardness test are shown in Table 1. The table
clearly show an improvement in hardness after hardening, and a decrease in hardness is observed
with tempering. From the results the maximum hardness of 152 HBN was obtained at 8500C
hardening followed by the as receive carbon steel of about 131 HBN. The annealed carbon steel has
the lowest hardness value of about 102 HBN.
Table 1. Hardness property of the as-received and the heat treated specimen
Specimen Specification
As received
Hardening
Normalizing
Tempering
Annealing
Hardness
131
152
111
119
102
Impact Test. Table 2 shows the energy recorded after an impact test for as-received specimen was
57.3 J, while the heat treated specimen such as hardening, annealing, normalizing and tempering
has the impact energy 64.1 J, 55.6 J, 65 J, and 65.4 J respectively. From the impact test result, the
tempered specimen has the highest impact energy just slightly greater than the normalized specimen
which means that the tempered specimen can absorb more energy before failure more than the rest
of the specimens and the annealed specimen has the lowest energy to absorb before failure.
Table 2. Impact test of the as-received and the heat treated specimen
Specimen Specification
As received
Hardening
Normalizing
Tempering
Annealing
Toughness (J)
57.3
64.1
65
65.4
55.6
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Fatigue Result. Summary of fatigue test results used to compare between the heat treatment effects
are presented in Table 3. The result shows that the endurance limit for normalized steel is higher
than that of annealed one. As normalized materials have higher yield stress as compared to annealed
one, the endurance limit is also higher than that of annealed one. Here for normalized treatment,
fatigue limit comes around 604 MPa for stress level at 1.1 × 103 cycles. But for annealed one, it
came 604 MPa at 1.4 × 103 no. of cycles. From the result at 604 MPa, the specimen with the highest
numbers of cycle is the tempered at tempering temperature of 2000 C. When a load of about 1208
MPa was added there is a decrease in the number of cycle, while tempering exhibit the highest
number of cycle but at the increment of load it decreases rapidly.
Table 3. Fatigue test results with different heat treatment conditions for SAE 1035 Steel specimens
Specimen Specification
As received
Normalizing
Tempering
Annealing
Stress (MPa)
604
1208
1812.3
604
1208
1812.3
604
1208
1812.3
604
1208
1812.3
No. of Cycles to Failure
1.3 x 103
0.7 x 103
0.3 x 103
1.1 x 103
0.4 x 103
0.2 x 103
2 x 103
0.8 x 103
0.1 x 103
1.4 x 103
0.3 x 103
0.1 x 103
Tensile Result. The resulting engineering Load – Displacement curves obtained from the test are
shown in Fig. 9 to Fig.13 for annealed, normalized, tempered and hardened and also the as receive
specimens respectively. The test data are plotted against engineering stress (load) vs. engineering
strain (elongation). Fig. 9 and Fig. 10 show that yield stress and ultimate tensile stress is more for
normalized as compared to annealed treatment. This enhancement may be due to the fact that the
cooling process is influenced by the cooling rate used for these treatments. Normalizing cooling rate
compared to annealing is faster, because in normalizing cooling process is done by air cooling and
in annealing this is done by furnace cooling. Due to this, more refine grains are obtained as
compared to the annealed one which induces more strength and less ductility in the material [23].
Fig. 9. Load – Displacement curve for the annealed specimen
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Fig. 10. Load – Displacement curve for the normalized specimen
Fig. 11. Load – Displacement curve for the tempered specimen
From Fig. 11 compared to the normalizing and annealing treatments, tempering has higher value
due to the more refined grains, as the specimens were subjected to austenizing, quenching and then
tempering [14]. Another mechanical property variation which should be in consideration is
ductility. During the tensile test, materials are elongated and then broken due to tensile stress.
According to the tensile test of tempering 200 0C for higher time we get a result of 18 mm/mm % of
elongation and for lower time 11 mm/mm, having less ductility as compared to the normalizing and
annealing. From this we can conclude that when strength is induced in the materials, it makes the
material more hard which gives the result of less ductility in the material.
Fig. 12. Load – Displacement curve for the as receive specimen
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Fig. 13. Load – Displacement curve for the hardened specimen
In case of as-received specimen, the ultimate tensile strength, yield strength are 121MPa and
91.5MPa respectively, whereas the percentage elongation is 22mm/mm. Comparing the as-receive
with the hardened heat treated specimen, the hardened specimen has the highest ultimate tensile
strength while the tempered specimen have the highest yield strength and the percentage elongation.
Modulus of Resilience. Table 4 shows the result of the modulus of resilience of the as received and
the heat treated specimens. The Modulus of Resilience is the amount of energy stored in stressing
the material to the elastic limit. Therefore the as receive specimen will have more stored energy
when stressed to its elastic limit, followed by the hardened specimen which is slightly greater than
the annealed specimen and the tempered specimen has the least stored energy when stress to its
elastic limit more than the other specimen. The as received Specimen has a value of 2939N /m2,
1413N/m2 for the Annealed specimen and 1303N/m2, 910N/m2 1450N/m2 for the normalized,
tempered and the hardened specimen respectively.
Table 4. The modulus of resilience of the as received and the heat treated specimen
Specimen Specification
As received
Annealing
Normalizing
Tempering
Hardening
Modulus of Resilience
(N/m)
2939
1413
1303
910
1450
Young’s Modulus. Table 5 Shows the young’s modulus of the as-received and the heat treated
specimen. From the figure, the annealed specimen has a young’s modulus value of 399.5 MPa
followed by the hardened specimen with the value of 250.9 MPa and that of the tempering,
normalized and as-receive specimen are 267 MPa, 203 MPa and 116.3 MPa respectively. The
annealed specimen with the value of 399.5MPa can withstand more load than the other specimen in
its elastic limit while the as receive specimen will withstand a lesser load to the other specimen.
Table 5. The Young’s Modulus of the as received and the heat treated specimen
Specimen Specification
As received
Annealing
Normalizing
Tempering
Hardening
Young’s Modulus
116.3
399.5
203.8
267.1
250.9
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Ultimate Tensile Strength. Table 6 shows the ultimate tensile strength of the as-received and the
heat treated specimen. Ultimate tensile strength (UTS), is the maximum stress that a material can
withstand while being stretched or pulled before failing or breaking. From the figure, the annealed
specimen having an ultimate strength of 118.6 MPa, and the as-receive specimen have an ultimate
strength of 121 MPa while the normalized, the tempered and the hardened specimen has the
ultimate strength of 104.4 MPa, 95.5M Pa and 179.6 MPa respectively. It can be seen from the
figure that the hardened specimen has the highest value of ultimate strength of 179.6MPa and the
tempered specimen has the lowest value with 95.5MPa. Therefore, the hardened specimen will
withstand more stress than the other specimen when stretched or pulled before failing or breaking,
followed by the as-received, annealed and the normalized specimen and the tempered specimen will
withstand the least stress than the other specimen when stretched or pulled.
Table 6. The Ultimate Tensile Strength of the as received and the heat treated specimen
Specimen Specification
As received
Annealing
Normalizing
Tempering
Hardening
Ultimate tensile strength
(MPa)
121
118.6
104.4
95.5
179.6
Percentage Elongation. Table 7 shows the percentage elongation of the as-received and the heat
treated specimen. The percentage elongation for the as receive and the heat treated specimen are as
follows: as-received 22%, Annealing 20.8%, Normalizing 24.3%, Tempering 24.6%, Hardening
12%. The tempered specimen having the highest elongation will be more ductile than the specimen
slightly greater than the normalized specimen and the hardened specimen will be less ductile among
the other specimen.
Table 7. The Percentage elongation of the as received and the heat treated specimen
Specimen Specification
As received
Annealing
Normalizing
Tempering
Hardening
Percentage elongation (%)
22
20.9
24.3
12
24.6
Percentage Area Reduction. Table 8 shows the percentage area reduction of the as-received and
the heat treated specimen. The as-received specimen have a percentage area reduction of 66%, the
Annealed specimen 81.5% while that of Normalized, Tempered and Hardened specimen are 59.1%,
78.9% and 75% respectively. The Annealed specimen have the highest percentage area reduction
followed by the Tempered specimen while the Normalized specimen have the least percentage area
reduction. Therefore, the Annealed specimen will tend to have more increase in length during the
necking period while the Normalized specimen with the least percentage area reduction will have
lesser length increment during the necking period than the other specimen.
Table 8. The Percentage Area Reduction of the as received and the heat treated specimen
Specimen Specification
As received
Annealing
Normalizing
Tempering
Hardening
Percentage Area Reduction (%)
66
81.5
59.1
78.9
75
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Conclusion
In this study effect of different heat treatment process such as annealing, tempering, normalizing
and hardening on mechanical properties such as impact, fatigue, hardness and tensile strength was
investigated. From this study, it may be concluded that the increase of fatigue strength is directly
proportional to increase in tensile strength. The best results are obtained for the specimen tempered
at 200 0C. These specimens have also shown the highest endurance limit. This is perhaps due to the
fact that these specimen possess vary high strength with significant ductility. So, as far as fatigue
strength is concerned, the tempering may be regarded as the best possible heat treatment operation.
Among the heat treatment techniques, tempering has got the most significant effect on fatigue life.
The changes caused by normalizing are mainly due to the increase in tensile strength in comparison
with annealing. Tempering treatment improves fatigue behavior and also other mechanical
properties. It can also be concluded that the material having higher UTS bear higher endurance
limits as compared to materials with other treatments. Comparing the mechanical properties of
tempering specimen with hardened sample, it was found that there was increase in young’s modulus
and hardness.
Acknowledgement
The authors wish to acknowledge the support given by Alo Francis of Materials and Metallurgical
Engineering Department, Obafemi Awolowo University, Ife for providing facilities for this
research. Also, the authors would like to thank the authors whose references are used for this
research.
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